In today's rapidly advancing optoelectronics industry, vertical cavity surface emitting lasers, also known as VCSELs, are preferred as optical sources. A VCSEL is a semiconductor microlaser diode that emits light in a generally cylindrical beam. The beam is emitted vertically from the surface of the substrate on which it is fabricated and a VCSEL offers significant advantages when compared to the edge-emitting lasers currently used in the majority of fiber optic communications devices. Vertical cavity surface emitting lasers are promising emitters for fiber data communication in the speed range of 100 Mbs and 1 Gbs. They enable high performance systems in Gigabit Ethernet, Fiber Channel, and ATM markets, for example.
A VCSEL is an extremely small laser which consists of two mirrors sandwiching an active region. The total thickness of the layers which make up the laser may be on the order of three microns or less. The mirrors reflect the light generated in the active region back and forth. The back and forth reflection results in “stimulated emission” providing emitted light at a single wavelength or color. Such “coherent” emission is the hallmark of lasing.
VCSELs are favored because of the ease of their manufacture, the repeatably of the manufacturing process used to form the VCSELs, the reduced substrate area each VCSEL requires, and the superior uniformity of VCSELs formed within the same substrate. Additionally, VCSELs can be produced in a two-dimensional array configuration on a common substrate and can be tested at the wafer level. Furthermore, the circular and low divergence output beams of VCSELs, eliminate the need for corrective optics and provide a high transmission speed with low power consumption at a relatively low cost. A principal characteristic of a VCSEL is that it emits light beams vertically rather than laterally, i.e., in a direction normal to the P-N junction and the surface of the semiconductor substrate on which the VCSEL is fabricated.
Because of the ease of their manufacture, the superior uniformity of VCSELs formed within the same substrate, and increased demands to integrate multiple data signals into a small area, tightly packed arrays of VCSELs formed within a substrate have become favored in today's optoelectronics industry. Each of the VCSELs in the fixed array may be coupled to an optical transmission medium such as an optical fiber and may each deliver an optical data signal. The optical data signal carries important information and is typically converted back to an electrical signal thereafter. It is useful to monitor and control the optical power of the light emitted by the VCSEL to provide the data signal. When the VCSELs are formed in close proximity in a tightly packed array, it is difficult to individually monitor the optical power of each VCSEL. Monitoring is typically performed by monitoring the primary light beam that is emitted by a VCSEL, after it reflects off another surface. For an array of tightly-packed VCSELs, only one of the VCSELs is typically monitored during operation due to space constraints and also because attempting to monitor each VCSEL using a dedicated monitor photodetector is very expensive.
VCSELs may be fabricated on one surface of a substrate such as a gallium arsenide (GaAs) wafer or another wafer chosen to be transmissive to light emitted by the VCSEL, particularly long wavelength (LW) VCSELs. It is therefore desirable to monitor the VCSEL by monitoring light emitted by the VCSEL, and which travels through the substrate and exits the opposed surface of the substrate. If the opposed surface of the substrate is a rough surface, however, light emitted from the backside of the VCSEL and travelling within the substrate, may continuously scatter in the substrate when it encounters the backside. Furthermore, if the VCSELs are arranged in an array on a common substrate, light emitted from multiple unintended VCSELs may exit the substrate and be sensed by a photodetector intended to monitor a single VCSEL. Light from the various VCSELs may exit at various angles, particularly when the surface is a rough surface. Therefore, there are difficulties associated with monitoring a VCSEL using backside monitoring.
It would therefore be desirable to provide an apparatus to accurately monitor the light of a designated VCSEL formed on the top surface of a substrate, by using a photodetector disposed opposite the bottom surface of the substrate.